A post-mixed burner is a burner wherein fuel and oxidant are delivered in separate passages to a point outside the burner, such as a furnace, where the fuel and oxidant mix and combust. One often used arrangement of passages employs a central tube for delivery of fuel encompassed by an annular passage for delivery of oxidant. The oxidant in the annular passage may be the major oxidant for the burner or it may be a small oxidant stream employed for flame stabilization purposes.
A phenomenon which sometimes occurs with burners is a flameout wherein the flame or combustion reaction is extinguished. Flameout is a very dangerous conditions because fuel and oxidant are continuously delivered to the combustion zone, in, for example, a furnace, and if there is no combustion reaction occurring to consume these combustibles, the fuel and oxidant may build up to hazardous levels. For this reason the flame in a burner is generally continuously monitored by a flame detection device which is also in contact with the fuel and oxidant supply systems. Should the flame monitor fail to detect flame, indicating a flameout, it will shut off the fuel and oxidant supply streams and thus avoid the hazardous buildup of an explosive mixture in the furnace. In a burner having concentric fuel and oxidant passages the flame detector is can be positioned so that it sights down through the central tube. This arrangement provides a simple and convenient method to detect the flame without the complexity of a separate flame detector built into the burner. An often used type of flame detector is an ultraviolet light detector.
A recent significant advance in the field of post-mixed burners is the aspirating burner developed by Dr. John E. Anderson which is disclosed in U.S. Pat. No. 4,378,205, and U.S. Application Ser. No. 428,013. In this way the momentum of the oxidant and thus of the combustion reaction is preserved and heat is delivered evenly throughout the furnace. The aspirating burner is characterized by developing a combustion reaction having a dilute flame which does not emit a strong ultraviolet light. Depending on the design and operation of the burner, the intensity of the flame signal may fall below a minimum value to provide a steady ultraviolet signal to satisfy the flame detector. The flame detector thus reads no flame and shuts off the fuel and oxidant supply. This results in a time consuming restart of the burner and an inefficient combustion process.
Other factors which may affect the flame detector and result in nuisance false flameout readings include soot or other opaque substances within the furnace due to dirty fuels or incomplete combustion and a low reflecting central tube due to a soot or corrosion covered surface. Such a surface does not reflect a requisite amount of light for the flame detector to function properly.
These other factors which adversely affect the reliability of a flame detector exacerbate the flame monitoring difficulties discussed above for the aspirating burner.
One possible way of overcoming the problem of a false flameout reading is to increase the intensity of the flame by diverting the direction of the fuel and/or oxidant so that they mix and combust close to or right at the tip of the burner. However, this possible solution to the problem has severe drawbacks because the characteristics of the flame, i.e. flame shape, direction, etc. are significantly altered. This may have a detrimental effect upon the efficiency and quality of the combustion process.
It is thus desirable to provide a flame detection device which can reliably monitor a dilute flame under all furnace conditions without significantly altering the characteristics of the flame.
It is therefore an object of this invention to provide a flame signal enhancer for use with a post-mixed burner.
It is another object of this invention to provide a flame signal enhancer for a post-mixed burner which has increased reliability and will avoid a false flameout reading.
It is a further object of this invention to provide a flame signal enhancer for a post-mixed burner which will avoid a false flameout reading while not significantly altering the characteristics of the flame.
It is a still further object of this invention to provide a flame signal enhancer for a post-mixed burner which will avoid a false flameout reading despite a dilute flame.